Gumbel softmax trick with beam search

[Akella17]

Is it possible to use Gumbel softmax trick with a beam search decoder in the existing implementation of Texar. I want to differentiate through the generated beam search samples into the generator network.

[ZhitingHu]

1. The current beam-search implementation does not support customization with decoder Helpers, though I think it's possible to refactor beam-search to enable the nice feature.

2. Here is a nice paper on direct optimization of beam search with continuous approximation: https://arxiv.org/pdf/1708.00111.pdf

Any contributions to 1) or 2) are welcome!